Excess mortality in patients with chronic kidney disease stage 5 is an important unsolved problem. Annual mortality of patients with chronic kidney disease stage 5 is about 10 to 20%. The parathyroid hormone (PTH) seems being a factor responsible for the excess mortality in patients requiring hemodialysis as a recent huge study demonstrated a J-shaped association between PTH and mortality. Consequently, parathyroid hormone (PTH) has been described as a uremic toxin with multiple systemic effects including bone disorders (renal osteodystrophy), myopathy, neurologic abnormalities, anemia, pruritus, and cardiomyopathy. Hyperparathyroidism is common in CKD and results in significant morbidity and mortality if left untreated. Low as well as high PTH levels measured by current PTH assays are associated with a progression of cardiovascular diseases and substantially increased all-cause mortality in patients on hemodialysis (Floege J. et al, ARO Investigators. Serum iPTH, calcium and phosphate, and the risk of mortality in a European haemodialysis population. Nephrol Dial Transplant. 2011; 26:1948-1955; Torres P A et al, Calcium-sensing receptor, calcimimetics, and cardiovascular calcifications in chronic kidney disease. Kidney Int. 2012; 82:19-25; Souberbielle J C et al. in Parathyroid hormone measurement in CKD. Kidney Int. 2010; 77:93-100)
Thus, guidelines have been established aiming to keep PTH in concentrations associated with the lowest morbidity and highest survival. The Kidney Disease Outcomes Quality Initiative (KDOQI) guidelines recommend measuring regularly PTH concentrations of patients with chronic kidney disease (CKD) and adjusting the patients' medication (e.g. vitamin D, phosphate binders, calcimimetics) such that plasma PTH levels are kept within a target range in accordance with the stage of CKD (e.g., 150 to 300 ng/L in patients with CKD stage 5). If pharmacological approaches do not work adequately, parathyreodectomy may be considered.
Secondary hyper-parathyroidism may also occur as an adaptive response to deteriorating renal function when circulating 1,25-dihydroxy vitamin D decreases as early as in stage 2 of CKD and continues to fall as the glomerular filtration rate (GFR) decreases. Chronic kidney disease is associated with a progressive loss of 1α-hydroxylase activity, because of functional reasons such as enzyme inhibition by hyperphosphataemia, hyperuricaemia, metabolic acidosis and sometimes also 25-hydroxyvitamin D deficiency. More important is, however, simply the loss of healthy renal tissue—and hense 1α-hydroxylase—explaining functional reduction of 1α-hydroxylase activity in CKD. As GFR decreases below 60 mL/min/1·73·m2 phosphate is retained which stimulates directly or via the klotho/FGF23 system secretion of PTH. Additionally the 1,25-dihydroxy vitamin D deficiency contributes in this situation to an increased secretion of PTH, since PTH secretion/gene expression in the parathyroid glad is negatively controlled by 1,25-dihydroxy vitamin D.
Hypocalcaemia develops as the GFR decreases below 50 mL/min/1·73·m2, stimulating a secretion of parathyroid hormone (PTH) from cells of the parathyroid gland into the blood circulation. In the intact form human parathyroid hormone (hPTH) consists of a single polypeptide chain having 84 amino acids and a molecular weight of ca. 9500 Dalton (see SWISS-PROT: P01270, PTHY-HUMAN). With disease progression, intact hPTH(aa 1-84) half-life increases and immunoreactive C-terminal fragments of the hormone tend to accumulate in serum. A chronic elevation of parathyroid activity then results in bone loss, fractures, vascular calcification, cardiovascular disease, and hence an increased cardiovascular mortality (cf Fraser W D, Hyperparathyroidism. Lancet. 2009; 374:145f).
Part of the problem with the use of PTH measurements has been confusion concerning the interpretation of the assays utilized. The measurement of PTH in blood has evolved since the early 1960s when RIAs were first developed for measurement of PTH (Berson S A et al, Proc Natl Acad Sci USA. 1963; 49:613-617). However, these first-generation assays proved not to be reliable owing to different characteristics of the antisera used and the realization that PTH circulates not only in the form of the intact 84-amino-acid peptide but also as multiple fragments of the hormone, particularly from the mid and carboxy (C)-terminal regions of the PTH molecule. The PTH peptide following secretion is degraded within minutes in the kidney in active and inactive fragments and the respective fragments have further varying half-lifes. A second generation of PTH immunoassays was developed using two antibodies one binding in the aminoterminal portion of the PTH peptide with the biologic activity and the other in its carboxyterminal portion (John M R et al. (1999), J. Clin. Endocrinol. Metab., 84. 4287-4290; Gao P et al. 2000, Poster M455, ASBMR 22nd Annual Meeting; Roth H J et al. (2000), Poster P1288; 11th International Congress of Endocrinology, Sydney). However, there was still a discrepancy between measured immunoreactive PTH concentration in serum and clinical findings (Goltzman D et al, in Discordant disappearance of bioactive and immunoreactive parathyroid hormone after parathyroidectomy, J Clin Endocrinol Metab 1984, 58(1):70-75. Thus, a third generation of intact PTH assay has been developed which however fails to improve the diagnosis of bone diseases or other clinical signs of secondary hyperparathyroidism in uraemic patients (Brossard J H et al., Influence of glomerular filtration rate on non-(1-84) parathyroid hormone (PTH) detected by intact PTH assays, Clin Chem. 2000; 46:697-703). It seems meanwhile accepted that some immunoreactive PTH fragments have a biological activity comparable with intact PTH peptides whereas others such as hPTH(3-34) seem to inhibit the effects of parathyroid hormone (see EP-A 0 349 545; Schmidt-Gayk et al. (1999) Osteologie forum, 5, 48-58), Suva et al. (1987) Science, 237, 893ff; EP 0 451 867). It has further been postulated that large inactive but immunoreactive non-(1-84) PTH fragments lead to erroneous determinations (LePage R. et al. (1998) Clin. Chem., 44, 805-809). Additionally, dipeptidyl peptidase-4 (DPP4) is expressed on the surface of many cell types and a rather indiscriminate serine exopeptidase. This led to the hypothesis of PTH being a substrate of DPP4 or a similar exoproteinase while the utmost two N-terminal amino acids are necessary for a cAMP-cyclase activity and binding of the PTH peptide to its receptor. Consequently, a also two-site immunoassays have been developed employing antibodies that can distinguish between aminoterminally “intact” PTH peptide chains and PTH peptides that are missing one or two amino acids at the utmost aminoterminus (see WO 2001/44818 (Armbruster et al), WO 96/10041 (Mägerlein et al); WO 2003/03986 (Hutchison J S)).
The discovery of oxidized PTH peptide chains in serum samples of uraemic patients has further led to the development of an immunoassay for determination of non-oxidized PTH (1-84) and biologically active fragments thereof (WO 2002/082092). Thus, there are plethora of immunoassays available for measuring parathyroid hormone in plasma and concentrations of various “bioactive” PTH peptide embodiments which are in some patient groups similar and other patient groups noticeably different. It would therefore be desirable to obtain reliable information on the patients' PTH status which allows an adaptation of the medication of kidney patients to reduce morbidity and mortality (see also Sprague S M et al, The Case for Routine Parathyroid Hormone Monitoring, Clin J Am Soc Nephrol, October 2012, as doi: 10.2215/CJN.04650512e; Goldsmith D J A, ebuttal: The Case for Routine Parathyroid Hormone Monitoring Clin J Am Soc Nephrol 8: 319-320, 2013. doi: 10.2215/CJN.10231012). The state of the art PTH measurement therefore represents a problem.